1. Technical Field
The present invention relates to steering control devices, and in particular, to a steering control device for detecting a steering torque applied on a steering, and controlling current flowing to a motor for driving the steering.
2. Related Art
A motor-driven power steering control device including a torque sensor for detecting the steering torque applied on a steering system by a driver of a vehicle, and a low pass filter for removing high frequency components is conventionally known. In such a motor-driven power steering control device, a drive current of a drive motor for applying a steering auxiliary force with respect to the steering torque by the driver is obtained based on outputs from the torque sensor and the low pass filter.
For example, Japanese Unexamined Patent Publication No. 2009-132344 discloses a motor-driven power steering control device that addresses a current command value rapidly changing and the vehicle becoming unstable immediately after an abnormality of the steering torque is detected. Such motor-driven power steering control device includes a torque sensor for detecting the steering torque of the steering, a low pass filter for removing the high frequency components of the output from the torque sensor, a torque command calculation unit for calculating a current command value based on the output from the low pass filter, and the like. The motor-driven power steering control device increases the time constant of the low pass filter when the abnormality of the steering torque is detected.
It is also conventionally known to control the current flowing to the motor for driving the motor-driven power steering device based on a torque sensor for detecting the steering toque applied on the steering system by the driver of the vehicle and a vehicle speed sensor for detecting the vehicle speed.
In such motor-driven power steering device, if the motor is controlled to a direction of assisting the left turn even if the driver is turning the steering wheel to the right, this goes against the will of the driver thus causing the driver to feel a sense of discomfort. The reason for the occurrence of such abnormality may be the abnormality of the CPU that computes the command current for driving the motor based on the steering torque. As a measure for such abnormality, a technique of stopping the drive of the motor when the direction of the output of the torque sensor and the direction of the target value of the command current or the direction of the current flowing to the motor do not coincide.
For example, Japanese Unexamined-Open Patent Publication No. 2002-059855 discloses a motor-driven power steering device in an aim of performing abnormality determination at high accuracy even when a damping control and the like is being carried out, and reliably prohibiting the drive of an electric motor at the time of abnormality. Such motor-driven power steering device includes a control section for controlling the electric motor, and a drive prohibiting section for prohibiting the drive of the electric motor at the time of abnormality. The drive prohibiting section determines whether or not the magnitude and the direction of the steering torque and the magnitude and the direction of the electric motor current satisfy a predetermined prohibiting condition, integrates the time in which the predetermined prohibiting condition is satisfied, and prohibits the drive of the electric motor when the integrated time becomes greater than or equal to a predetermined time.
Japanese Unexamined-Open Patent Publication No. 2008-238843 discloses a motor-driven power steering device in an aim of prohibiting unnatural behavior of the steering. The motor-driven power steering device checks point coordinates of a combination of the steering torque and the motor current with a window time setting map to determine the window time. The motor-driven power steering device determines to stop the drive of the motor when the point coordinates continuously exist for the window time in a region corresponding to the determined window time, and stops the drive of the drive controlled motor.
Japanese Unexamined-Open Patent Publication No. 2006-051912 discloses a motor-driven power steering device in an aim of detecting the abnormal state more quickly even when the vehicle speed is large, and stopping the steering aid. Such motor-driven power steering device includes an assist map showing the correspondence relationship between the current target value and the steering torque, and outputs the current target value with reference to the assist map. The motor-driven power steering device includes a drive prohibiting map including the condition for stopping the steering aid as a predetermined drive prohibiting region, and determines whether or not to stop the steering aid with reference to the drive prohibiting map. The motor-driven power steering device selects the drive prohibiting map including a relatively wide drive prohibiting region only when the vehicle speed is large and the detection value of the motor current is small, and stops the motor when the current target value and the steering torque accord with the condition shown in the selected drive prohibiting map.
It is known to perform low pass filtering process on the output of the torque sensor as disclosed in Japanese Unexamined-Open Patent Publication No. 2009-132344. The high frequency component is thereby removed from the output of the torque sensor, and the high frequency component of the current flowing to the motor is also reduced. The vibration from the steering wheel is thus also alleviated, enhancing the comfortableness in driving.